Reality of Force. 391 
third Law of Motion tells us that force is always dual, and that 
to every action there is always an equal and contrary reaction, 
he goes on thus (art. 289): — " 'Do you mean to tell me/ 
said a medical man of the old school, ' that if I pull a ' sub- 
ject ' by the hand, it will pull me with an equal and opposite 
force ? ' When he was convinced of the truth of this state- 
ment, he gave up the objectivity of force at once." 
I cannot help thinking that this gentleman was not only a 
doctor of a very old school, but a very old doctor of any school; 
for I have in vain endeavoured to discover in this rebellious be- 
haviour on the part of subjects anything which could constitute 
any reason — physical, metaphysical, logical, or otherwise — for 
believing or not believing in the objectivity of matter. Why 
a thing should be real if left to itself, but become unreal and 
fictitious if it is opposed to something equal to itself, is a puzzle. 
We may conceive our doctor arriving at very singular conclu- 
sions if he carried out the same principle consistently. Thus the 
celebrated Irishman, who complained that it was not his fall that 
hurt him, but the stopping so suddenly, might have been told 
that he was in error: his fall was an objective action, but when 
it was stopped by the equal and opposite action of the earth it 
became a mere rhetorical figment. Again, if a gentleman 
squeezes a lady's hand, that is an objective fact; but if she 
squeezes his in return, then it becomes merely a subjective 
impression. This would not interest the doctor, but may be 
a useful hint to younger practitioners. If it be objected that 
in these cases the opposition is temporary, while in the case 
of force it is permanent, I would reply that permanence, 
teste Prof. Tait himself, is a proof of reality rather than the 
reverse. And I still inquire in what way the existence of two 
equal and opposite causes proves the unreality of either or 
both of them. 
There is one deduction from the new view, which Prof. 
Tait makes himself (art. 297), and which deserves notice. He 
observes that " equivalent quantities must always be expressed 
by equal numbers when both are measured in terms of the 
same system of units. It appears therefore, from the con- 
servation of energy directly, that potential energy must, 
like kinetic energy, be of dimensions [ML 2 T -2 ]. Now it is 
impossible to conceive of a truly dormant form of energy 
whose magnitude should depend in any way on the unit of 
time ; and we are therefore forced to the conclusion that 
potential energy, like kinetic energy, depends (in some as yet 
unexplained, or rather unimagined, way) upon motion. . . The 
conclusion appears inevitable that, whatever matter may be, 
